Solar module power generation is a method of generating power by directly converting inexhaustible solar energy into electric energy. Viewed as a technology for significantly alleviating energy issues, solar module power generation has therefore been intensely researched and developed in recent years, and its market has also expanded considerably.
Currently, single-crystal silicon substrates and polycrystal silicon substrates are widely used for the substrates of solar modules. A solar module that uses a single-crystal silicon substrate or the like is formed of a number of substrates called solar cells of a size of some tens of centimeters square. The multiple solar cells forming the solar module are interconnected by collector wires for collecting the electrical energy generated by the individual solar cells. Molten phase bonding with solder is widely adopted for the connections between the solar cells and the collector wires. The collector wire is known as an interconnector for current collection and is formed from solder-coated flat copper wire. Flat copper wire is generally produced by rolling round wire to form flat wire (metal tape). Owing to such a production method, the flat copper wire can be produced in a thin and elongated shape.
On the other hand, the solar module is an energy device that outputs electric power as electric current. From this it follows that the cross-sectional area of the interconnector for current collection and the area of the bonding surface between the interconnector for current collection and the solar cells need to be determined with consideration to the amount of current flowing through the interconnector for current collection.
In order to bond the interconnector for current collection to a solar cell, it is necessary to perform processing for heating and joining the interconnector for current collection and the solar cell by liquid-phase bonding, followed by cooling to room temperature. In this process, thermal stress occurs because of the difference between the coefficient of thermal expansion of the silicon that is the main component constituting the solar cell and the coefficient of thermal expansion of the copper that is the main component constituting the interconnector for current collection. The typical coefficients of linear thermal expansion of metal and silicon in the vicinity of room temperature are 16.6×10−6 (K−1) for copper, 19×10−6 (K−1) for silver, 25×10−6 (K−1) for aluminum, and 3×10−6 (K−1) for silicon. When copper and silicon are bonded at 200° C., a length difference of about 0.26% arises. And this length difference produces thermal stress and warping between the copper and the silicon. As pointed out earlier, the ratio between the coefficient of thermal expansion of copper and the coefficient of thermal expansion of silicon is large, at around 5 fold, so that the thermal stress produced may deform or break the solar cell. On the other hand, in order to cope with tight silicon material supplies and lower the cost of solar modules, the thickness of substrates used in solar cells is being reduced. For example, very thin silicon substrates of 180 μm-order thickness have come to be used in solar cells. Breakage of solar cells by thermal stress has therefore become a still greater problem.
Attempts have been made to overcome this problem by softening the interconnector for current collection (see, for example, Non-patent Document 1). In order to deal with the problem caused by difference in coefficient of thermal expansion between metal and silicon, it is important to soften the interconnector for current collection, i.e., to lower its Young's modulus and yield stress. In general, 0.2% proof stress is usually used as the definition of yield stress. Also in the case of an interconnector for current collection, strain can be expected to be induced on the order of around 0.2%. So lowering the 0.2% proof stress is to allow the metal side to yield, experience thermal stress, and warp. The method generally used to soften a metal is to lower dislocation density by annealing. However, reduction of 0.2% proof stress by anneal-softening has its limit, so that it has been difficult to keep pace with further thickness reduction of solar cell substrates. In light of this, various technologies have been proposed for improving current-collection interconnector structure and packaging, and also for collection system control (see, for example, Patent Documents 1 to 3).
The invention taught by Patent Document 1 relieves stress by forming wavy zones in the longitudinal direction of the interconnector for current collection. Further, the invention taught by Patent Document 2 reduces thermal stress in the cooling process following current-collection interconnector bonding, by forming non-contact regions not formed with electrodes at desired intervals in the longitudinal direction of the solar cell electrodes. In addition, so as to lower 0.2& proof stress, the invention taught by Patent Document 3 aligns the crystallographic orientation (plating wire axis direction) of the conductor core in the (211) plane at a ratio of 30% or greater, thereby decreasing solar module warping.
A technique that mitigates thermal stress by modifying the connecting structure between the solar cell and the interconnector for current collection is very effective. However, the technique taught by Patent Document 1 increases the length of the required interconnector for current collection, so that it increase the materials cost of the interconnector for current collection and may also increase its electrical resistance. Further, since the techniques taught by Patent Documents 1 and 2 reduce the bonding area between the solar cell and the interconnector for current collection, connection resistance rises and the electrical resistance of the bond region (notch region) may also increase. Therefore, aside from such techniques, a strong need is felt for improvement of the mechanical properties of the interconnector for current collection by making the material itself of the interconnector for current collection lower in Young's modulus and lower in yield stress. It should be noted that a similar problem is liable to arise also in various types of solar modules other than solar modules that use polycrystal silicon substrates because the materials of the solar module material and the current collection conductor are different.
On the other hand, wire-bump bonding has recently been proposed in which bumps composed of metal are solder-connected on top of a wafer and used to bond metal wire or metal tape (see, for example, Patent Document 4). As thermal stress is apt to arise also in the case of performing solder connection, the same problem as pointed out above regarding the interconnector for current collection of a solar module is liable to occur.
Moreover, the metal foil used in a flexible circuit board is one example of utilizing aggregate structure to control the mechanical properties of a packaging electrical conductor other than an interconnector for current collection for a solar module (see, for example,
Patent Documents 5 to 7). The method set out in Patent Document 5 requires the (200) plane intensity (I) determined by X-ray diffraction of the rolled surface to be as follows with respect to the (200) plane intensity (I0) determined by X-ray diffraction of copper fine powder: I/I0>20 or greater. This is to improve the fatigue property, i.e., the property when the foil is repeatedly bent. Further, the method set out in Patent Document 6 requires that the metal foil consist of a metal having a crystal structure of cubic system and that when cut in the thickness direction from the ridge at a bend, the principal axis of the cross-section of the interconnector, assuming the zone axis to be [001], lie in a plane included in the range of from (20 1 0) to (1 20 0) in the direction of rotation from (100) to (110). In addition, the metal foil set out in Patent Document 7 requires that the area fraction of crystal grains present within 15 degrees of the angle formed between the [100] direction of the crystal and the rolling direction be 80% or greater and the maximum grain diameter be 5 μm or less. However, there is a limit to 0.2% proof stress reduction with the metal foil set out in Patent Document 7 in which only the crystal grains oriented in the rolling direction are small in diameter.